Carbon and Nitrogen Turnover in Sediments of the Soyang Lake Reservoir

TERRECO Cluster H-03

Lake Soyang is the largest reservoir in South Korea and has an important meaning for the supply of drinking water to the Seoul metropolitan area. It is, therefore, highly relevant to understand ongoing process in the lake in regard to the development of its water quality. Lake Soyang is a warm monomictic lake which has only one turnover period for water layers in the winter season. The water body is stratified by density difference due to water temperature normally during June to November. During stratification oxygen depletion occurs in the hypolimnion through decomposition of organic matter in lake sediment. Under these conditions, phosphate which is usually the limiting factor for phytoplankton population and density in Korean fresh water, and gases such as N2O and methane gas which are regarded as global warming gas can be released from sediment to the water and atmosphere. Korea has a monsoon climate so that more than half of annual precipitation is concentrated in the summer monsoon season. In this time, a large amount of terrestrial materials, including nutrients (nitrogen, phosphorus) from farming areas, and organic matter mainly from forest sites flows into the lake in runoff from the watershed. We hypothesize that the processes controlling the water quality in Lake Soyang are strongly related to the monsoon climate. During the monsoon period lake water quality is controlled by external processes, which are the input of materials from the watershed. During the non-monsoon period with very low discharge in the upstream rivers, the lake is almost disconnected from the catchment and internal processing of materials occurs. The main purpose of this study is to understand the extent to which catchment processes affect the nutrient dynamics in the lake. To this end, the chemical composition of the sediments and of the water column will be investigated over annual cycles.
We hypothesize that materials released from the sediment (e. g. phosphate, methane and hydrogen sulfide), which are formed under anoxic condition and become distributed to the entire water body during the mixing period, can impact water quality during the dry season. In contrast, external input during the monsoon season with high loads of phosphorus, nitrogen and carbon will control internal process such as the density at the thermocline/pyconcline and also the species succession of phytoplankton and the sediment composition during the monsoon season.